pretend that your life is all about the match.you have worked hard since u are small to pplay in the match,and to win it.u will lose everything if u lose that match.ur life will be a failure if u lose it<------MAYBE TOO SERIOUS
so ,what will u do to avoid it from happening?yes,concentrate.u dont want to lose ur time of training football before just not concentrate and lose the match.tell urself:play this match as good as possible,forget about everything but the match,just concentrate.do ur best.then after the match,u can think again other things(things that make u not concentrate :problem with friend/team-mate/family/gf.......)just keep 100% focus in the match
even if ur enemy/foe off the pitch,if he is ur team-mate on the pitch,he will assist u to score goal too.in other words,i mean thatwhen u play football, ure in another world.
before kick off,think:"this is football time,forget about everything,focus on the ball'then u wont be regret if u lose ..........if u have tried ur best and concentrate
actually this is just mental probelm,when u can overcome it since now.u wont have the same problem again in the future.try ur best,u will become automatically concentrate 100% on the pitch then
dont make concentration problem 'block' u from becoming a great player...good luck!!!!!!!
hope u understand